When you flex, no fibre actually shortens itself: the actin and myosin filaments merely slide past one another, dragging the two Z-lines closer. The harder the effort, the more motor units are recruited, the sarcomere visibly contracts and the EMG goes from scattered spikes to a dense train. Myasthenia gravis makes every repeat weaker as antibodies destroy the postsynaptic acetylcholine receptors; a cramp locks the muscle in contraction.
When you flex, no fibre actually shortens itself: the actin and myosin filaments merely slide past one another, dragging the two Z-lines closer. The harder the effort, the more motor units are recruited, the sarcomere visibly contracts and the EMG goes from scattered spikes to a dense train. Myasthenia gravis makes every repeat weaker as antibodies destroy the postsynaptic acetylcholine receptors; a cramp locks the muscle in contraction.
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